Heating means



y 3, 1966 M. RYDINGER ETAL 3,249,676

HEATING MEANS Filed Nov. 6, 1963 United States Patent 7 HEATING MEANS Mats Rydinger, Bengt Fredriksson, and Janis Blaus,

Vasteras, Sweden, assiguors to Allmann Svenska Elektriska Aktiebolaget, Vasteras, Sweden, a corporation of Sweden Filed Nov. 6, 1963, Ser. No. 321,862 Claims priority, application Sweden, Nov. 23, 1962, 12,581/ 62 Claims. (Cl. 13-26) For a fireproof material which is used in heating systems it is required for many purposes that it does not sinter together with a neighbouring fireproof material. Among these uses may for examples be mentioned exchangeable burner heads, measuring inserts with enclosed measuring device (e.g., thermostats), inspection stones, sealing doors in metallurgical furnaces and the lining in inductor units of channel type, which are exchangeable and are in contact with the lining of the furnace crucible. It has already been proposed to place an intermediate layer of mica or asbestos in the joint between the exchangeable and the stationary fireproof parts. This resists sintering only up to about 1200 C. Chrome spinel has also been suggested. For chrome spine], however, the heat resistance is limited to temperatures which lie below those occurring in many cases, so that the risk of sintering together in narrow joints cannot be avoided.

The present invention relates to a heating means comprising at least two ceramic parts which border on each I other, e.g., a metallurgical furnace with a ceramic lining and with an exchangeable inductor unit with a ceramic lining, in which means a heat resistant composition is arranged between the ceramic parts in order to prevent a sintering together of these, characterised in that the heat resistant composition comprises at least 85% by weight, preferably at least 95% by weight, magnesium oxide and chromium oxide, of which at least 90% by weight are linked to each other as magnesium chromite. The chemical composition of the magnesium chromite is MgO-Cr O The composition contains at the most 6% by weight each of free magnesium oxide, MgO, and chromium oxide, 050,. Most suitably the composition should consist wholly of magnesium chromite. In order that the sintering tendency caused by impurities shall be as small as possible, the composition should contain at the most 6% by weight of impurities. Impurities in the form of aluminium oxide and iron oxide should amount to at the most 3% by weight of each, and in the form of silicon dioxide to atthe most 2% by weight. The particle size of the composition is adapted to the intended layer thickness of the joint which will be made of the composition and to the method of application, e.g., painting or put-tying. If the composition is to be used for making a joint with a certain thickness it is suitable that 20-70, preferably 30-60% by weight of the particles have a size which is substantially equal to the intended thickness of the joint which is to be made, While the remaining particles have a size which is considerably less, preferably below about A of the intended joint thickness. If the joint which is to be made is to have a thickness of 1 mm. it is thus suitable that 2070%, preferably 30-60% by weight of the particles have a size of about 1 mm. and the rest of the particles a size which lies below about 0.25 mm. The coarser particles act as spacing elements between the ceramic parts and the finer particles fill up the interstices between the coarser particles.

In order that the com-position may be applied without difficutly at the intended place it may be mixed with a binder. As a binder is preferably chosen one which upon heating evaporates or decomposes without leaving any 3,249,675 Patented May 3, 1966- residue or a residue which does not promote sintering. The content of binder suitably amounts to at most 9% by weight. Examples of usable binders are, inter alia, cellulose derivatives such as ethylhydroxyethylcellulose, carboximethylcellulose and methylcellulose, polymerised vinyl compounds such as polyvinyl chloride and polyvinyl alcohol, starch, dextrin, different types of resinous binders such as melamine resins, urea resins and ester resins, etc.

An embodiment of the present invention will be described with reference to accompanying drawing, where FIGURE 1 shows a part of a furnace, e.g., a melting or holding furnace for iron, in the form of a section through an inductor unit along the line AA in FIGURE 2 and a part of the lining of the furnace body, and FIGURE 2 shows a section along the line B-B in FIGURE 1, the core and coil being removed. FIGURE 3 is a side view and FIGURE 4 a front view of such a holding furnace.

The inductor unit 1 has, as usual, an iron core 2, a coil 3, a melting channel 4, a lining 5 and a casing 6. Only a small part of the furnace body 7 is shown. The lining of the furnace body consists of an inner layer 8 of heat resistant sintered ceramic material and an outer layer 9 of non-sintered material in powdered form. An insulating mantle 10 of conventional ceramic material encloses the outer layer 9 and is enclosed in turn by a metallic casing 11. The linings 5 and 8 may each be of neutral, basic and acid type, and of mutually different types. The lining 5 may for example be basic, when the lining 8 is acid.

The outer layer 9 is reinforced by a ring-shaped plate 12 which supports a ring-shaped cooling channel 13 which is engaged by the outer layer 9. The cooling channel 13, the plate 12 and an outer wall 14 and ceiling 15 form a space 16 which serves as a drain-age channel for condensed water from the cooling channel 13 and for Water possibly leaking therefrom. Between the lining of the furnace body and the inductor unitis inserted a joint material 17 according to the invention..

The casing 6 of the inductor unit is surrounded by a wall 18, so that a space 19 is formed, which thus acts as drainage chamber. 20 which are attached by lugs 21 to the casing 6. The uppermost cooling pipe concentrates, as will be seen, the cooling efiiect upon the joint between the furnace body and the inductor unit. The latter is fixed to the furnace body by means of screws 22. In order to obviate a short-circuited secondary loop in the casing 6 and mantle 18 these parts are interrupted by an insulated gap 23 which is positioned below the zone of concentrated cooling.

In the simplified sketch according to FIGURES 3 and 4, two attached inductor units 1 and means for rotating the holding furnace are shown.

The joint 17 may be produced in different ways, of which some are exemplified below:

A composition consisting of 94 parts by weight of magnesium chromite with an average particle size of 0.3-0.35 mm., of which 45 parts by weight have a particle size of 0.84l.0 mm., and the rest a particle size less than 0.25 mm., is mixed with 6% by weight of polyvinyl chloride. In order to give the composition a suitable oonsistence for application it is mixed with a suitable amount of tetrahydrofurane. The composition is painted or puttied on each or both the surfaces on the lining bodies 5 and 8 which face each other so that a joint about 1 mm. thick is produced. The tetrahydrofurane evaporates rapidly from the joint. In connection with the heating of the furnace to operating temperature the polyvinyl chloride is decomposed to gaseous products 7 which escape.

There are several cooling pipes- Another usable composition may be produced by mixing 96 parts by weight of magnesium chromite with an average particle size of 0.55-.65 mm, of which 45 parts by weight have a particle size of 1.682.0 mm. and the rest a particle size less than 0.5 mm., with 4 parts -by weight of a urea resin. The compound is diluted with water to suitable consistency. It is then applied in a 2 mm. thick layer in the same way as previously described. Binder and diluting agent escape also in this case.

A further usable composition may be produced by mixing 99 parts by weight of magnesium chromite with an average particle size of 0.50-0.65 mm., of which 55 parts by weight have a particle size of 1.68-2.0 mm. and the rest a particle size less than 0.5 mm., with 1 part by weight of ethylhydroxyethylcellulose. The composition is diluted with water to suitable consistency. It is then applied in a 2 mm. thick layer in the way previously described. Binder and diluting agent escape in conjunction with the starting up of the furnace.

Even if the invention is described especially for the case where the heating means constitutes a furnace, it is obvious that it may with advantage be used in other heating means which include two adjacent ceramic parts, which must be prevented from sintering together.

We claim:

1. Heating means comprising at least two ceramic parts having surfaces turned towards each other, a heat and the percentage of silica in the composition at the most 2.

5. Heating means as claimed in claim 1, in which said heat resistant composition is arranged in the form of a layer between the ceramic parts and said heat resistant composition comprises 2070% 'by weight of particles of a diameter substantially equal to the thickness of the layer.

resistant composition arranged between and separating said surfaces for preventing said parts from sintering together, said heat resistant composition comprising at least 85% by Weight of magnesium oxide and chromium oxide, at least 90% by weight of the magnesium oxide and chromium oxide being linked to each other as magnesium chromite.

2. Heating means as claimed in claim 1, in which said heat resistant composition comprises at the most 6% by weight of chromium oxide in free form.

3. Heating means as claimed in claim 1, in which said heat resistant composition comprises at the most 6% by weight of magnesium oxide in free form.

4. Heating means as claimed in claim 1, in which said heat resistant composition comprises at most 6% by weight of impurities in the form of aluminium oxide, iron oxide and silica, the percentages of aluminium oxide and iron oxide in the composition being at the most 3 6. Furnace comprising a furnace body with a lining of heat resistant material on the inside, at least one inductor unit replaceably attached to the furnace body with a lining of heat resistant material on the inside, said linings facing each other in the assembled furnace, a layer of a heat resistant composition between the lining of the furnace body and the lining of inductor unit for preventing said linings from sintering together, said heat resistant composition comprising at least by weight of magnesium oxide and chromium oxide, at least by weight of the magnesium oxide and chromium oxide being linked to each other as magnesium chromite.

7. Furnace as claimed in claim 6, in which said heat resistant composition comprises at the most 6% by weight of chromium oxide in free form.

8. Furnace as claimed in claim 6, in which said heat resistant composition comprises at the most 6% by weight of magnesium oxide in free form.

9. Furnace as claimed in claim 6, in which said heat resistant composition comprises at most 6% by weight of impurities in the form of aluminium oxide, iron oxide and silica, the percentages of aluminium oxide and iron oxide in the composition being at the most 3 and the percentage of silica in the composition at the most 2.

10. Furnace as claimed in claim 6, in which said heat resistant composition comprises 2070% by weight of particles of a diameter substantially equal to the thickness of the layer.

References Cited by the Examiner UNITED STATES PATENTS 2,599,566 6/1952 Magri 106-66 3,100,237 8/1963 Rydinger et al 13-26 3,154,624 10/1964 Dolph et al. 13-35 RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner. 

1. HEATING MEANS COMPRISING AT LEAST TWO CERAMIC PARTS HAVING SURFACES TURNED TOWARDS EACH OTHER, A HEAT RESISTANT COMPOSITION ARRANGED BETWEEN AND SEPARATING SAID SURFACES FOR PREVENTING SAID PARTS FROM SINTERING TOGETHER, SAID HEAT RESISTANT COMPOSITION COMPRISING AT LEAST 85% BY WEIGHT OF MAGNESIUM OXIDE AND CHROMIUM OXIDE, AT LEAST 90% BY WEIGHT OF THE MAGNESIUM OXIDE AND CHROMIUM OXIDE BEING LINKED TO EACH OTHER AS MAGNESIUM CHROMITE. 